LOS ANGELES/KYIV/MOSCOW - US President Donald Trump disclosed Wednesday that Russian President Vladimir Putin told him that Moscow would "have to respond" to the massive Ukrainian drone attacks on Russia's strategic air bases days ago.

Trump wrote in a post on his Truth Social account that he finished a phone call with Putin and discussed the attacks on June 1. It was the first time the US president publicly talked about the operation that Ukrainian President Volodymyr Zelensky touted being directed by himself.

"We discussed the attack on Russia's docked airplanes, by Ukraine, and also various other attacks that have been taking place by both sides... President Putin did say, and very strongly, that he will have to respond to the recent attack on the airfields," Trump wrote.

He said the Wednesday call lasted approximately one hour and 15 minutes, noting "It was a good conversation, but not a conversation that will lead to immediate Peace."

Zelensky: Russia's ceasefire memorandum ‘ultimatum’

On Wednesday, the Interfax-Ukraine news agency reported that Zelensky described Russia's memorandum on ceasefire as an "ultimatum" to Ukraine.

"A memorandum of understanding should involve two parties, not just one side that demands something. Therefore, it cannot be called a memorandum. It is, in fact, an ultimatum from the Russian side," Zelensky said.

He emphasized that some of Russia's requirements, particularly regarding the territorial claims, could not be fulfilled.

Speaking on Russia's call for Ukraine to adopt non-aligned status, Zelensky said that in 2014, when developments in Crimea and the Donetsk and Luhansk regions started, Ukraine had a non-aligned status defined by law.

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Regarding joining NATO, according to Zelensky, "this issue depends on the allies."

Ukraine received the memorandum from Russia during the bilateral talks were held in Istanbul on June 2.

Crimean Bridge explosion

Also on Wednesday, the Kremlin confirmed that Ukrainian forces had attempted to attack the Crimean Bridge, but failed to cause any damage.

"There really was an explosion, nothing was damaged, and the bridge is working," Kremlin spokesperson Dmitry Peskov told reporters, responding to a question on Ukraine's attempted attack on Tuesday.

Traffic on the Crimean Bridge was briefly halted on Tuesday at 15:23 Moscow time (1223 GMT), according to the bridge's official Telegram channel.

The Security Service of Ukraine said Tuesday that it had struck the Crimean Bridge for the third time using underwater explosives, as about 1,100 kilograms of TNT-equivalent explosives were placed under the bridge and detonated.

According to a video published by the Russian Federal Security Service on the same day, a detained suspect of the Crimean city admitted that he had made a bomb to commit a terrorist attack on orders from Kyiv. 

'500-for-500' prisoner exchange

Separately, the Ukrinform news agency reported that the Ukrainian president announced Wednesday that Ukraine and Russia have agreed to carry out a prisoner exchange under the "500-for-500" formula this weekend.

"The Russian side has informed us that this weekend it will be ready to hand over 500 individuals ... Accordingly, we will be ready to exchange the same number of people," Zelensky said after consultations between Kyiv and Moscow regarding the swap.

He noted that Ukraine has not yet received the lists of individuals to be exchanged.

Ukraine and Russia have agreed on another major prisoner exchange during the second round of direct talks in Istanbul on Monday.

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The delegations held their previous negotiations on May 16, which resulted in a "1,000 for 1,000" prisoner exchange between the parties.